1. Field of the Invention
The present invention relates to a steer-by-wire drive control system such as a steer-by-wire steering device built in a vehicle and particularly to means for updating the home position of an operating element.
2. Description of the Prior Art
Referring to FIGS. 5 and 6, an example of a conventional steer-by-wire steering device will be described below. FIG. 5 shows the structure of a conventional steer-by-wire steering device and FIG. 6 is a graph showing the relation between an external force applied to the tire and a reactive force applied to the steering wheel.
As shown in FIG. 5, this steer-by-wire steering device comprises: a steering wheel 1 as an operating element; an operating angle sensor 2 which senses the operating condition of the steering wheel 1; an operating element actuator 3 as a force feedback actuator which applies an external force for force feedback to the steering wheel 1; a tire reactive force sensor 5 for sensing the reactive force from a road surface which acts on a vehicle""s steering system 4 as an operated device; a steering actuator 6 as an actuator for driving the operated device which applies an external force for steering to the steering system 4; a first controller 7 (CPU) which receives output signals a and f from the operating angle sensor 2 and the steering system 4 respectively and controls the operating element actuator 3; and a second controller 9 which receives output from the tire reactive force sensor 5 and a signal b for information on the operating condition of the steering wheel 1 and controls operation of the steering actuator 6.
The steering wheel 1 is designed to be able to be turned clockwise and counterclockwise. The steering system 4 of the vehicle is designed to be able to move the tires 8 to the right and the left by the same amount from the home position of the system. The home position of the steering wheel 1 for force feedback control is adjusted with reference to the home position of the steering system 4 of the vehicle so that the vehicle can go straight ahead on a flat road surface.
When the driver turns the steering wheel 1 clockwise or counterclockwise while driving, the operating direction and angle of the steering wheel are detected by the operating angle sensor 2, which in turn outputs a steering signal a depending on the operating direction and angle of the steering wheel 1. The first controller 7 receives this steering signal a and generates and outputs an information signal b for indicating the operating condition of the steering wheel 1, which depends on the signal a. This information signal b drives the steering actuator 6 to laterally move the vehicle""s steering system 4 in the direction corresponding to the operating direction of the steering wheel 1, by the angle of rotation equivalent to the operating angle of the steering wheel 1, so that the vehicle curves to the right or left.
As the vehicle curves, a reactive force (cornering force) from the road surface which depends on the vehicle""s speed, steering angle and other factors acts on the tires 8 and the tire reactive force sensor 5 outputs a tire reactive force detection signal c which depends on the reactive force. The second controller 9 receives the tire reactive force detection signal c and outputs a control signal f. The first controller 7 receives the control signal f and generates and outputs a drive signal d for the operating element actuator 3 which depends on the signal f. This drive signal d drives the operating element actuator 3 and, as shown in FIG. 6, an external force proportional to the reactive force which acts on the tires 8 is given to the steering wheel 1. In this way, the driver can get the same kind of driving feeling as when he or she drives a vehicle with a mechanical steering device.
However, over time, the home position of the steering wheel 1 for force feedback control may become out of alignment with the home position of the steering system 4 built in the vehicle because of irregular tire wear or irregular wear of a steering system mechanical component.
If, for some reason, the home position of the steering wheel 1 for force feedback control deviates from the home position of the steering system 4 built in the vehicle, when the steering wheel 1 is held in its home position, the vehicle would curve in the direction opposite to the deviation; on the other hand, when the steering wheel 1 is operated in a manner to suit the running condition of the vehicle, the steering wheel 1 should be off its home position, or on the deviation side, the driving feeling would worsen and also, even if the steering wheel 1 is turned by the same angle of rotation clockwise and counterclockwise, the vehicle would not curve by the same angle of rotation clockwise and counterclockwise, impairing the vehicle""s driving safety and operability.
The conventional type of steer-by-wire steering device has no means to automatically compensate for a discrepancy between the home position of the steering wheel 1 for force feedback control and the home position of the steering system 4 and, therefore, cannot prevent the above-mentioned inconvenience.
The present invention has been made to solve the above problem inherent to the prior art and provides a steer-by-wire drive control system with a high operability and a high durability which automatically compensates for a discrepancy between the home position of the steering wheel for force feedback control and the home position of the steering system built in the vehicle.
In order to solve the above problem, according to one aspect of the present invention, a steer-by-wire drive control system which comprises: an operating element which can be operated in a first direction and a second direction; a sensor which detects an operating condition of the operating element; an operating element actuator which applies an external force to the operating element; and a controller which controls operation of the operating element actuator with reference to a memorized home position, wherein the controller calculates frequencies of a signal outputted from the sensor and updates the operating element""s home position memorized in the controller to the operating element""s position where the signal is most frequently outputted, as a new home position of the operating element.
Taking a steer-by-wire steering device as an example, the steering wheel is operated with reference to the home position of the steering system (operated device) regardless of the home position of the steering wheel (operating element) for force feedback control in order to let the vehicle run along a desired course. In other words, if, for some reason, there is a discrepancy between the steering wheel""s home position for force feedback control and the steering system""s home position, in his/her attempt to let the vehicle run along a desired course, the driver controls the steering wheel with reference to a position deviated from its home position instead of with reference to the home position of the steering wheel. In addition, during driving, in which the steering wheel is repeatedly turned clockwise and counterclockwise, the steering wheel operating angle corresponding to the steering system""s home position is most frequently used and the larger the steering wheel operating angle is, the less frequently it is used.
Thus, since the controller repeatedly calculates frequencies of signals outputted from the steering wheel operating angle sensor to get information on the current actual home position of the steering wheel, the steering wheel operating angle (position) where the signal is most frequently outputted is re-memorized in the controller as a new home position of the steering wheel to make the steering wheel""s home position for force feedback control agree with the steering system""s home position. Once both the home positions have agreed, operation of the operating element actuator (force feedback actuator) can be controlled with reference to the new home position. As a consequence, it is possible to prevent worsening of the vehicle""s driving feeling or deterioration in the vehicle""s driving safety due to a discrepancy between the steering wheel""s home position and the steering system""s home position.
In order to solve the above problem, according to another aspect of the present invention, the controller updates the operating element""s home position memorized in the controller when a difference between the operating element""s home position memorized in the controller and the calculated new home position reaches a prescribed value.
An advantage of this arrangement is that the operability and driving safety are improved because the operating element""s home position is updated before the driver begins to feel deterioration in the vehicle""s operability.
In order to solve the above problem, according to a further aspect of the present invention, a steer-by-wire drive control system comprises: an operating element which can be operated in a first direction and a second direction; a first sensor which detects an operating condition of the operating element; an operating element actuator which applies an external force to the operating element; an operated device which is operated in a first direction and a second direction from a home position by controlling the operating element; a second sensor which detects a condition of the operated device; an operated device actuator which drives the operated device; a first controller which controls operation of the operating element actuator with reference to a memorized home position of the operating element; and a second controller which receives information on the operating condition of the operating element from the first controller and controls operation of the operated device actuator, wherein the first controller receives a signal outputted from the second sensor to detect a home position of the operated device and updates the operating element""s home position memorized in the first controller with reference to the home position of the operated device.
An advantage of this arrangement is explained next taking a steer-by-wire steering device as an example. As stated earlier, while a vehicle is running, a reactive force from the road surface, which depends on the vehicle""s speed, steering angle and other factors, acts on the tires. The reactive force which acts on each tire is the smallest when the vehicle is running straight ahead, namely the vehicle""s steering system (operated device) is in its home position, and increases as the steering angle of the steering system increases. Therefore, the controller can calculate the home position of the steering system from signals which it receives from the tire reactive force sensor (second sensor).
As stated earlier, while the vehicle is running, the steering wheel is operated with reference to the home position of the steering system (operated device) regardless of the home position of the steering wheel (operating element) for force feedback control. The steering wheel operating angle (position) corresponding to the steering system""s home position is re-memorized in the controller as a new home position of the steering wheel so that the steering wheel""s home position agrees with the steering system""s home position. As a consequence, worsening of the vehicle""s driving feeling or deterioration in its driving safety due to a discrepancy between the steering wheel""s home position and the steering system""s home position can be prevented.
In order to solve the above problem, according to a further aspect of the present invention, the first controller updates the operating element""s home position memorized in the first controller when a difference between the operating element""s home position memorized in the first controller and the operated device""s home position reaches a prescribed value.
An advantage of this arrangement is that the operability and driving safety are improved because the operating element""s home position is updated before the driver begins to feel deterioration in the vehicle""s operability.